It used to be that Billboard's Hot 100 Singles charts was compiled using sales figures. Then radio airplay came into it. Now Billboard knows that if their chart is going to be an accurate reflection of the musical zeitgeist, it has to consider all the ways people access songs. That includes YouTube.
The Hot 100 Singles Chart began using data from streaming music services a little while ago. Now they're incorporating YouTube plays into their weighting. As Editorial Director Bill Werde says, "There are lots of different ways a song can be a hit."
This goes a long, long way to explaining why Baauer's "Harlem Shake" is now #1.